(1) Technical Field
The present invention relates to solid-state curved focal plane arrays. More specifically, the present invention relates to a system and method for making solid-state curved focal plane detector arrays that may be matched to a given optical system.
(2) Background
Conventional detector arrays are flat; therefore, a matching flat optical focal surface is required in designing optical systems. To meet this requirement, a high price is often paid in terms of size, weight, complexity, cost, and performance. The human eye provides a natural example of an imaging system whose detector (the retina at the back of the eye) is curved to correspond with its focal surface (the lens). The Schmidt camera (an ultra-fast telescope with a film plate holder instead of an eyepiece at its focus) is a classic example of a system which will benefit from a curved focal plane array (CFPA). Other examples of systems which could benefit from curved focal plane arrays include: wide-field rover cameras, star-tracker cameras, and far ultra-violet (UV) instruments.
Curved focal plane detector arrays simplify optical designs by reducing the number of optical elements by up to a factor of four, increasing reliability of the system, and increasing the margins of tolerance in optical design. Therefore, the production of lighter and more efficient optical systems for airborne or space-borne missions is possible. In addition, there are commercial applications in the areas of charged-particle detection systems and miniaturized cameras.
Curved focal plane detector arrays offer other advantages. While other aberrations depend on the stop and conjugate positions within an optical system, field curvature generally depends only on the basic constructional parameters of the system and the throughput. Thus, field curvature is very difficult to change, and can be regarded as intrinsic to an optical system. The designer has more degrees of freedom in controlling other aberrations than in controlling field curvature. Curved focal plane detector arrays offer a way out of this dilemma by permitting the designer to concentrate on the correction of other aberrations rather than having to abandon a certain design approach due to excessive field curvature. By such innovation, for example, an instrument having the capabilities of National Aeronautics and Space Administration's (NASA's) Multi-angle Imaging SpectroRadiometer (MISR) could be miniaturized from the size of an office desk to the size of a suitcase. One method of improving optical systems is using fiber-optic faceplates for matching a curved focal surface to a flat detector. Inherent limitations of this approach include: mismatch of fiber-optic faceplate and detector resolution; limited transmission wavelengths of fiber (e.g., opaqueness at UV wavelengths); and alignment complexities.
Many NASA missions rely on imaging and spectroscopy instruments. The NASA vision of smaller, lower cost, and more frequent missions mandates the miniaturization of instruments. For optical systems, the designer generally must introduce one or several additional optical elements to flatten the inherently curved focal surface. This complication and its corresponding penalty in mass, performance, and cost can be avoided if the detector itself is made to conform to the naturally-curved focal surface of a much simpler optical design.
In practice, micro-channel plates (MCPs) have been made in curved configurations, making the NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) and the Alice instrument in the European Space Agency's Rosetta mission possible. However, in addition to the difficulties and cost associated with fabrication of these especially-curved MCPs, these instruments come with the higher mass and power overhead of MCPs.
Both the FUSE and Rosetta missions benefit from curved detector technology. These missions are designed to perform detailed UV spectroscopy by collecting photons diffracted from curved gratings with detectors of matched curvature. For these missions MCPs with the needed curvature were provided. In FUSE, the MCP detectors are curved and positioned to match the Rowland circle of the gratings. The Alice instrument (an ultraviolet imaging spectrometer) in the Rosetta mission is similar in concept and smaller in size. The very light-weight Alice instrument, approximately 2.2 kilograms (kg), credits its reduced mass and cost to the innovations of this mechanical design, wherein the input surface of its MCP has been made curved to match the 150 millimeter (mm) diameter Rowland circle for optimum focus.
Development of simple and low-cost technologies for fabricating curved solid-state detector arrays, whose curvature can be matched to the requirements of a specific system enables significant advances. Re-tooling the very-large-scale-integration (VLSI) industry from today's flat planar substrates to new production in curved space is a major challenge. Fabrication of CFPAs on individually curved substrates using e-beam lithography followed by processing, while an effective option, is very costly.
One example of a flat focal plane arrays is a charge-coupled device (CCD). CCDs are the detectors of choice for many NASA applications because back-illuminated CCDs potentially have the highest quantum efficiency in the ultraviolet and visible parts of the spectrum. However, CCDs require removal of the silicon substrate, i.e., thinning, and a surface treatment of the backside of the CCD to allow detection of UV photons.
Back-illuminated standard CCDs must be thinned to a thickness of approximately 10 to 20 μm corresponding to the epitaxially grown silicon layer (referred to as the epilayer), a property of the original CCD design. A natural consequence of having such a thin structure laced with electronic circuitry is that it will wrinkle with a height variation of 30 to 50 micrometers (μm). This is undesirable for astronomy applications which require focal plane flatness within fractions of a micron. Furthermore, such a membrane is fragile and prone to fracture when subjected to repeated flexing, which is inevitable in the temperature cycling of operational instruments. To obtain a thinned, flat, focal plane array, a rigid support can be attached to the CCD prior to the thinning process or after the thinning process. Attachment of a support substrate to the CCD front-side prior to thinning has been implemented by a number of groups, e.g., “Bump Bonded Back Illuminated CCDs” by M. P. Lesser, Ann Bauer, Lee Ulrickson, Proceedings of SPIE, vol. 1695, p. 508 (1992) and “Flat, Thinned Scientific CCDs” by Rusty Winzenread, Proceedings of SPIE, vol. 2198, p. 886 (1994) and “Thinned Charge Coupled Devices with Flat Focal Planes for UV Imaging,” by Todd J. Jones, Peter W. Deelman, S. Tom Elliott, P. J. Grunthaner, R. Wilson, and Shouleh Nikzad, Proceedings of the SPIE, vol. 3965, p. 148 (2000).
A number of UV-enhancement treatments have been demonstrated for back-illuminated CCDs: see for example, “Recent Developments in Large Area Scientific Grade CCD Image Sensors,” J. Janesick, T. Elliot, R. Bredthauer, J. Cover, R. Schafer and R. Varian, Proceedings of SPIE, vol. 1071 (1989), “Growth of a Delta-doped Silicon Layer by Molecular Beam Epitaxy on a Charge-coupled Device for Reflection-limited Ultraviolet Quantum Efficiency,” M. E. Hoenk, P. J. Grunthaner, F. J. Grunthaner, M. Fattahi, H. F. Tseng, and R. W. Terhune, Applied Physics Letters vol. 61, p. 1084 (1992), and “Enhancing Back-Illuminated Performance of Astronomical CCDs,” M. P. Lesser and V. Iyer, Proceedings of SPIE, vol. 3355, p. 446 (1998).
Delta-doping processes have also been developed to address UV enhancement CCDs as described in the above referenced paper, “Growth of a Delta-doped Silicon Layer by Molecular Beam Epitaxy on a Charge-coupled Device for Reflection-limited Ultraviolet Quantum Efficiency,” M. E. Hoenk, P. J. Grunthaner, F. J. Grunthaner, M. Fattahi, H. F. Tseng, and R. W. Terhune, Applied Physics Letters vol. 61, p. 1084 (1992). Delta-doped CCDs exhibit uniform and stable 100% internal quantum efficiency in the visible and ultraviolet regions of the spectrum without hysteresis, see “Delta-Doped CCDs as Stable, High Sensitivity, High Resolution UV Imaging Arrays,” S. Nikzad, M. E. Hoenk, P. J. Grunthaner, R. W. Terhune, R. Wizenread, M. Fattahi, H-F. Tseng and F. J. Grunthaner, Proceedings of SPIE, vol. 2217, p. 355 (1994).
A number of thinning approaches have been developed over the years at different companies and institutions. A summary and historical account of these approaches can be found in “Flat, Thinned Scientific CCDs,” Rusty Winzenread, Proceedings of SPIE, vol. 2198, 886 (1994). The early thinning efforts resulted in free-standing membranes, see “Improved Uniformity in Thinned Scientific CCDs,” Rusty Winzenread, Pat Chaio, Weng-Lyang Wang, and Lloyd Robinson, Proceedings of SPIE, vol. 1161, (1989). Several groups were able to produce thinned membranes supported on rigid substrates, e.g, Radio Corporation of America (RCA) as described in “High Sensitivity Charged-Coupled Device (CCD) Imagers for Television,” Eugene D. Savoye, Donald F. Battson, Thomas W. Edwards, William N. Henry, Donald R. Tshudy, and L. Franklin Wallace, Proceedings of SPIE, vol. 501, p. 32 (1984), University of Arizona as described in “Bump Bonded Back Illuminated CCDs,” M. P. Lesser, Ann Bauer, and Lee Ulrickson, Proceedings of SPIE, vol. 1656, p. 508 (1992) Tektronics as described in “Back Illuminated 2048×2048 Charge-coupled Device Performance,” K. Gladhill, M. Blouke, P. Marriott, T. Houk, B. Corrie, and H. Marsh, Proceedings of SPIE, vol. 1656 (1992), EG&G Reticon as described in “Flat, Thinned Scientific CCDs,” Rusty Winzenreqad, Proceedings of SPIE, vol. 2198, p. 886 (1994) and Science Applications International Corporation (SAIC) as described in “Megapixel CCD Thinning/Backside progress at SAIC,” A. R. Schaefer, R. H. Varian, J. Cover, and R. Larsen, Proceedings of SPIE, vol. 1147, p. 165 (1991).
What is needed is a system and method for producing curved focal plane detector arrays that may be matched to a given optical system.